1. Technical Field
The present invention relates to an AD conversion apparatus and a control method.
2. Related Art
The S/N ratio of a successive approximation AD converter depends on the accuracy of its internal comparator. The magnitude of the noise generated by the comparator depends on the common potential of the input end of the comparator. Therefore, the S/N ratio of a successive approximation AD converter can be improved by setting the common potential of its internal comparator to an optimal value.
An successive approximation AD converter equipped with a charge redistribution type differential DA converter is already known (Patent Document No. 1). This type of AD converters uses a comparator to compare a positive voltage with a negative voltage in the differential output voltage outputted from the differential DA converter in the successive approximation, thereby detecting DAC data at which the differential output voltage becomes substantially 0.    Patent Document No. 1: U.S. Pat. No. 6,400,302
The differential DA converter outputs a differential output voltage that changes between the positive side and the negative side with the common potential as the center. Therefore, the differential DA converter can maximize the dynamic range of the differential output voltage by setting the midpoint potential of the positive reference potential and the negative reference voltage to the common potential.
In a successive approximation AD converter having such a differential DA converter, however, the comparator compares the positive voltage and the negative voltage in the differential output voltage outputted from the differential DA converter. Therefore, when changing the common potential of the comparator for the purpose of reducing the noise from the comparator, the common potential of the differential DA converter should be changed as well. For this reason, with a successive approximation AD converter equipped with a differential DA converter, it has been difficult to reduce the noise from the comparator as well as widening the dynamic range of the differential DA converter.